1. Field of the Invention
The present invention relates in general to surface units for downhole pumping units and in particular to a surface power unit for generating a series of pulses of pressurized fluid to actuate a downhole pump.
2. Description of the Prior Art
Low pressure non-flowing wells account for the vast majority of the oil wells in the United States. There are various means available for pumping these non-flowing oil wells, including subsurface pumps which are electrically or hydraulically actuated. One problem which is common to both of these types of subsurface pumps is that a separate energy transmission path is required for supplying the actuating energy to the pump.
There have been several attempts to provide a rodless subsurface pump system which does not require a separate energy transmission path for activating the pump. Such a pump system typically includes a surface unit which is connected to the subsurface pump by a single fluid conduit. This surface unit activates the subsurface pump by applying pressure to the fluid in the conduit so as to compress a spring means in the pump and displace a slidable piston, thereby drawing fluid from the well into a pump chamber. When the surface unit releases the fluid pressure, the spring means of the downhole pump displaces the piston and lifts the fluid in the pump chamber into the fluid conduit. Such systems are disclosed in U.S. Pat. Nos. 2,058,455, 2,123,139, 2,126,880, and 2,508,609.
Several problems, however, are inherently associated with these pressure-activated subsurface pump systems. Since thousands of feet typically separate the surface unit from the downhole pump, considerable work is done compressing fluid in the conduit, ballooning the conduit, and moving fluid to compress the subsurface pump spring. The energy applied to the fluid in the fluid conduit system is much greater than the energy supplied to the subsurface pump. In these systems, considerably more energy is consumed in compressing the spring and ballooning the conduit than is used to lift the fluid. Thus, these systems are energy inefficient.
It would be desirable to provide a subsurface pump which has a relatively long stroke length such that more fluid could be produced for a given amount of energy input. Early subsurface pumps utilized strong helical compression springs as a means for lifting the fluid into the fluid conduit. Such springs severely limited the maximum stroke length which could be attained. Later subsurface pumps utilized an inert gas pressurized chamber which functioned as the spring means. When pressure was applied to the fluid conduit, a piston compressed gas within the chamber and, when the fluid pressure was relieved, the gas expanded to lift the fluid into the conduit. Such a subsurface pump is disclosed in U.S. Pat. No. 4,013,385.